Elastomer-reinforced polymers of vinyl aromatic compounds, such as styrene, alpha-methylstyrene and ring-substituted styrene, have found widespread commercial use. For example, elastomer-reinforced styrene polymers having discrete particles of cross-linked elastomer dispersed throughout the styrene polymer matrix can be useful for a range of applications, including, without limitation, food packaging, office supplies, point-of-purchase signs and displays, housewares and consumer goods, building insulation, and cosmetics packaging. The incorporation of an elastomer into the styrene matrix results in improvements in a range of physical and mechanical properties (e.g., impact strength) and collectively these polymers are termed high-impact polystyrenes (HIPS).
The utility of a particular HIPS depends on the polymer having some combination of mechanical, thermal, and/or physical properties that render the material suitable for a particular application. These properties are related in part to the extent of incorporation of the elastomeric material into the polymer matrix. Many factors during polymerization can affect the properties of polymer. Once such factor is the degree of crosslinking in the rubber phase, which may result in decreased impact resistance and environmental stress cracking resistance (ESCR), which may be reflected by a lower swell index.
Some crosslinking may be desired for low to medium viscosity rubber to stabilize the rubber particle morphology through the devolatilization process. However, excess crosslinking may alter the elasticity of the elastomer phase and be detrimental to the final properties of HIPS, such as impact strength and environmental stress cracking resistance.
Accordingly, there exists an ongoing need for systems and processes for providing HIPS having desired impact strength and ESCR. Desirably, such systems and processes enable production of HIPS having a suitable degree of crosslinking in the rubber phase, while maintaining a desirable color, as will be described in more detail hereinbelow.